1. Field of the Invention
The present invention relates to an observation optical system that combines light fluxes from plural display elements to present one enlarged combined image, in particular, relates to an observation optical system suitable for an image display apparatus such as a head-mounted display (HMD).
2. Description of the Related Art
For observation optical systems used for image display apparatuses such as HMDs, image presentation with a wide view angle is required to enable natural observation of images and to increase realistic sensation. Moreover, it is desirable that the observation optical system for the image display apparatus being mounted on an observer's head be small and thin.
Some of such small and thin observation optical systems capable of presenting images with a wide view angle use plural display elements displaying original images for mutually different view angles, and form one image by combining plural enlarged images corresponding to the original images to enable observation of an enlarged combined image. The use of the plural display elements enables reduction in size and thickness of an optical system for each of the plural display elements, which realizes a wholly small and thin observation optical system capable of presenting images with a wide view angle.
Such image display apparatuses have been disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820, 10-246865 and 09-166759 and Japanese Patent No. 3524569.
The observation optical systems disclosed in Japanese Patent Laid-Open Nos. 07-274097 and Japanese Patent No. 3524569 provide therein a V-shaped mirror symmetric with respect to a visual axis. In these observation optical systems, a light flux from an original image displayed on one display element is reflected at one reflective surface of the V-shaped mirror to be introduced to an exit pupil, and a light flux from an original image displayed on another display element is reflected at another reflective surface of the V-shaped mirror to be introduced to the exit pupil.
The observation optical system disclosed in Japanese Patent Laid-Open No. 11-326820 includes a prism formed into a shape symmetric with respect to a visual axis. A light flux from an original image displayed on one display element enters the prism through one entrance surface thereof facing the one display element, is reflected at a first surface which is a transmissive and internal total reflective surface of the prism, is reflected at another reflective surface, and then is transmitted through the first surface to be introduced to an exit pupil. Further, a light flux from an original image displayed on another display element enters the prism through another entrance surface thereof facing the other display element, is reflected at the first surface, is reflected at still another reflective surface, and then is transmitted through the first surface to be introduced to the exit pupil.
The observation optical system disclosed in Japanese Patent Laid-Open No. 10-246865 also includes a prism formed into a shape symmetric with respect to a visual axis. In this observation optical system, first and second entrance surfaces of the prism respectively facing first and second display elements are used also as reflective surfaces introducing light fluxes from the second and first display elements to an exit pupil.
The observation optical system disclosed in Japanese Patent Laid-Open No. 09-166759 includes prisms disposed on a visual axis at positions near and far from an exit pupil. The prism near the exit pupil reflects a light flux from an original image displayed on one display element twice to introduce it to the exit pupil, and the prism far from the exit pupil reflects a light flux from an original image displayed on another display element twice to introduce it to the exit pupil through the prism near the exit pupil.
However, each of the observation optical systems disclosed in Japanese Patent Laid-Open Nos. 07-274097, 11-326820 and 10-246865 and Japanese Patent No. 3524569 is not capable of overlapping the light fluxes from the respective original images with each other at a boundary part of the enlarged images mutually combined. Therefore, when an observer's eye is rotated, a joint line of the enlarged images becomes noticeable in the boundary part, which disturbs natural image observation.
Moreover, in the observation optical system disclosed in Japanese Patent Laid-Open No. 09-166759, since all the light flux from the prism far from the exit pupil is transmitted through the prism near the exit pupil, the prism far from the exit pupil is enlarged, which causes an increase in size of the entire observation optical system.